(i) Field of the Invention
The present invention relates to an image forming apparatus which detects occurrence of positional deviation and magnification deviation among images.
(ii) Background Art
There has been conventionally known an image forming apparatus which forms a color image using a plurality of image forming devices for forming images of each color. For example, a color laser printer is provided with four photoconductors. Toner images of magenta (M), cyan (C), yellow (Y), and black (Bk) are respectively formed on the four photoconductors. The four color toner images are superimposed and transferred onto an inter-transfer unit to form a toner image of four colors. The toner image of four colors is transferred onto a recording medium, so that a color image corresponding to the toner image of four colors is formed on the recording medium.
In such an image forming apparatus, due to errors in laser optics, used for exposure of the photoconductors, and errors in assembling the photoconductors, positional deviation and magnification deviation among the four color toner images may occur on the inter-transfer unit. Then, the positional deviation and magnification deviation among the four color toner images may also occur on the recording medium.
In order to correct the positional deviation and magnification deviation, there has been known an image forming apparatus which prints a reference line on the recording medium using one image forming device, prints a target line using another image forming device, and detects the positional deviation and magnification deviation between the images based on the deviations between the reference line and the target line (for example, see Publication of Unexamined Japanese Patent Application No. 9-109453). If the positional deviation and magnification deviation between the images are detected, the positional deviation and magnification deviation can be solved by correcting a laser scanning starting position and a scanning range used for exposure of the photoconductors.